checked his research notebook and was surprised to find that the atypical cell cultures had all been cultured to approximately their 40th doubling while younger cultures never exhibited the same problems. Furthermore, conditions were similar between the younger and older cultures he observed--same culture medium, culture containers, and technician. This led him to doubt that the manifestations were due to contamination or technical error.

Hayflick next set out to prove that the cessation of normal cell replicative capacity that he observed